This invention is directed to improving the performance of honeycomb core structural panels by localized reinforcement and densification of the panel.
Lightweight, high strength, panels made up of a honeycomb core and one or two face sheets bonded thereto have come into widespread use in aircraft, space vehicles and the like. While the panels have great strength across a broad panel, difficulties are encountered when it becomes necessary to fasten other structures to the panels. For example, it is often necessary to have a circular access opening in a panel, covered by a circular cover overlapping the edges of the opening and fastened to the panel by fasteners such as bolts. The panel honeycomb core and face sheets do not have the strength or local resistance to deformation to accommodate the bolts and localized stress. Similar problems arise when it is necessary to fasten a circular flange to the face of a honeycomb panel.
These problems are especially significant where ducts or tubular casings are fabricated of honeycomb panels. Often, reinforcement of the ring-like end of the duct or a ring-like section along the duct is necessary, such as to provide attachment regions for duct support assemblies.
In the past, attempts have been made to improve the local strength and thermal transfer characteristics of honeycomb panels by drilling a ring of holes in the panel face sheet and injecting a synthetic resin potting compound to fill selected cells or groups of cells to locally strengthen the panel. Also, fasteners such as nuts for access cover bolts or the like may be embedded in the potting compound, as described, for example, by Moji et al in U.S. Pat. No. 4,716,067. While effective for some purposes, this method is not adaptable to all fasteners, requires considerable hand-work and care to properly fill the cells and adds considerable weight to the panel.
In other cases, a ring-like region of the honeycomb core is removed, prior to application of the face sheets, and a solid metal ring insert is placed in the cutout. The insert is typically diffusion bonded or adhesively bonded to the face sheets when those sheets are bonded to the core. While these solid inserts are effective with a great many fasteners, they tend to be heavy and add an undesirably amount of weight, which can be of critical importance in aircraft and space vehicle applications. Also, the metal inserts must have a thickness precisely matching the honeycomb core thickness. Thinner inserts allow localized crushing of the panel before the insert is encountered, while thicker inserts will cause local bulges in the panel which are undesirably in aerodynamic surfaces. Thus, special, tight tolerance, inserts must be made for every honeycomb core thickness.
Also, while the thermal insulating properties of honeycomb panels are highly desirable in many applications, in some cases it is necessary to improve heat transfer in a local area from a heat source on one side of the panel to a heat sink or the like on the other side of the panel.
Therefore there is a continuing need for improved ring-like inserts for honeycomb core panels which can increase the density, strength and thermal conductivity in local areas without excessively increasing the weight of the assembly and can be easily made to precisely match a variety of honeycomb core thicknesses.